xml-dev - Re: DTDs, W3 XML Schema, RELAX, or Schematron?

[Rick JELLIFFE <ricko@geotempo.com>]

Linda van den Brink wrote:
> 
> > > Or will DTDs become obsolete and will W3C XML Schema,
> > RELAX, or Schematron
> > > replace them?
> 
> Eric van der Vlist wrote:
> > MHO is that different validation techniques will be used for different
> > needs.
> 
> Then what's really interesting to me, is which validation technique will be
> used for which need, and why?
 
I think the key issues are:
 1) Casual or Q&D validation versus formal modeling
    The more one's need is for the former, the more that minimalism is a
virtue rather than a hinderance.  The extremes here are Schematron
(minimalist) and XML Schemas (not famously minimalist). DTDs and RELAX
in between.

 2) Validation of regular structures (e.g. susceptible to being modeled
by a simple grammar) or path-based components.  
The extremes here are Schematron and DTDs.  RELAX and XML Schemas are in
between (RELAX provides better selectors for structural types than DTDs
or XML Schemas; XML Schemas because it provides path-based constraints
on data values w.r.t. uniqueness and key references.)

 3) Document-centred or interface-centered.
The extremes here are Schematron at one end and RELAX and XML Schemas at
the other, with DTDs all forlorn.   If you need to generate optimal
interfaces (or optimal DOMs) then you need datatype information and
grammars provide a nice way to bundle record declarations together.  If
you merely want to describe and validate a document, and you have no
interest in generating optimal DOMs etc. then schematron is more useful.

There are lots of other considerations too. 

The paper is a little out-of-date on the edges already, but there is an
interesting article from SIGMOD characterizing 6 schema languages. 
RELAX is not included, but I think it is fair to say that, in the
context of the article, it would fit in somewhere between DSD and XML
Schemas.

See
 http://citeseer.nj.nec.com/lee00comparative.html
and click PDF on top right corner.

Cheers
Rick Jelliffe